The initial speed of the projectile and the gravity can be changed with sliders.
You can drag the far end of the bar to change the angle (it can be changed with the slider,too).
You can find out at what angle the projectle will reach maximum horizontal distance when falls back to the same height.
You can make your own demonstration:
1. Replace the blue line with a stick.
2. Prepare many strings with appropriate length (check out step 4),
3. Loosely tie those strings at equal distance to the stick. (it will be better if you make a circle at one end of the string and put those strings at equal distance)
4. Cut the strings so that the length L(i)=L*i*i;
The distance between string represents initial speed of the projectile (distance travel at unit time).